1. Field of the Invention
This invention relates to stabilized autodeposition coating baths for coating metal surfaces. In particular, the invention relates to a continuous autodeposition coating process which is economical and provides uniform coatings of high quality throughout the life of the bath.
In order to obtain an autodeposition coating bath having optimum coating efficiency, bath components consumed in the coating process must be replenished, and contaminants generated must be removed.
In the coating of steel surfaces, iron is a major contaminant, with typical iron losses to the processing bath of 20-40 mg Fe/ft.sup.2 (about 200-400 mg Fe/m.sup.2) of surface treated. In large-scale commercial applications, such losses can translate into an iron build-up in the coating bath of 100 or more grams per hour, to concentrations in excess of 3 gm/1. At these concentrations, the bath interferes with the coating process, and the entire bath, including expensive resin components present, must be discarded in favor of a fresh bath. The gradual build-up of iron within the bath also has the disadvantage of progressively altering the coating characteristics of the bath, and coatings obtained from the spent bath before discard generally vary significantly in quality from coatings obtained from a fresh bath.
2. Prior Art
None of a variety of methods for maintaining autodeposition baths at optimum efficiency has been entirely successful when applied to continuous coating operations. In particular, the addition of phosphoric acid to autodeposition baths to precipitate iron for iron control has generated a troubling sludge in some systems, which is difficult to continuously and completely remove. In addition to depleting the bath of polymer coating material, the generating sludge tends to contaminate the product film coatings, and adversely affect their corrosion-protection abilities.